Technical Field of the Invention
The present invention relates to the field of cosmetics, and more particularly to the field of skin.
The invention resides in the identification by the inventors of 25 genes the modulation kinetics or the degree of modulation of which in the epidermis following a challenge reflects the epidermal homeostasis of the skin and distinguishes between a skin termed “aged” and a skin termed “young”.
Description of Background and/or Related and/or Prior Art
The stratum corneum is the superficial layer of the epidermis situated at the interface between the organism and its environment. It is composed of corneocytes, which are anuclear cells resulting from the differentiation of epidermal keratinocytes. Corneocytes are rich in keratins and are surrounded by an impermeable lipid matrix. Because of its protein and lipid composition, the stratum corneum plays an essential role as a cutaneous barrier. It prevents the intrusion of microbiological agents and can preserve the hydration of the skin and thus of the body in general.
All physical or chemical challenges to the integrity of the stratum corneum result in an increase in water loss through the skin. Hence, the topical application of acetone or detergents causing superficial delipidation of the stratum corneum, or tape stripping, substantially increases the transepidermal water loss (Grubauer, G et al, 1989; Kuss, O. et al, 1998; Pinnagoda, J et al, 1990; Tanaka, M. et al, 1997; Zhai H. et al, 1998). Such alterations in fact perturb the overall epidermal homeostasis and induce therein a physiological response which aims to restore the integrity of the barrier function of the stratum corneum. Thus, keratinocytes from the superficial layers of the epidermis activate the extracellular secretion of lipid bodies in order to rapidly provide the skin with a fresh impermeable membrane and thus to reduce the superficial loss of water (Grubauer, G. et al 1987; Menon, G. K., 1985). Further, the proliferation of keratinocytes of the stratum basale is activated to replace cells of the stratum corneum which have been damaged (Proksch E., 1991; Wood, L. C L et al, 1992).
The “transepidermal water loss” (TEWL) measurement is the most routinely used measurement for evaluating the integrity of the stratum corneum's barrier function (Piepkorn, M. et al, 1994). The TEWL is measured using an evaporimeter. This apparatus has a moisture and temperature detector which enables it to measure the water evaporation gradient at the skin surface. However, TEWL measurements are subjected to a number of different factors which limit accuracy. The ambient temperature, hygrometry, air turbulence or pressure at which the apparatus is applied can induce substantial variations in these measurements (Barel A O et al, 1995). The state of stress of the volunteer during the measurements also has a non-negligible influence, and thus a sufficient rest time before taking a reading is advised elsewhere (Van S et al, 1994). Even in regions as alike as the forearm, randomization of the measurements has been shown to be vital, the TEWL being significantly increased on the dominant forearm (Treffel P et al, 1994). In this context, the development of a simple, reliable, reproducible and accurate method for evaluating the state of the epidermal barrier function and as a consequence the dynamics of its homeostatic function would be of particular advantage.
Regarding ageing of the skin and apart from the known consequences of age on the relief of the skin, aged people report many problems. These problems have their origin in an alteration in the barrier function and the epidermal homeostasis of their skin. It is in particular often observed in aged persons that there is a great tendency towards chronic xerosis, characterized by an increased loss of water through the skin. In fact, the stratum corneum of aged skin has a decreased intercellular lipid content compared with young skin, particularly during the winter period. This change in the composition of the stratum corneum perturbs its physico-chemical cutaneous barrier properties. The stratum corneum becomes much more sensitive with age to physical or chemical challenges such as tape stripping or the application of acetone (Ghadially R et al, 1995) and its permeability to medication and in particular to hydrophilic compounds is decreased. Further, the severity of contact allergies is exacerbated on aged skin because of a slower and thus less frequent epidermal turnover (Piaserico S ea, 2004). Finally, the rate of recovery of the barrier function after alteration of the stratum corneum slows down with age, leading to the assumption that the homeostatic function of the epidermis is in dysfunction (Denda, M., 2002; Ghadially, R. et al, 1995; Leveque, J. L., 2001). However, despite these functional perturbations due to skin ageing, it has been shown that the thickness of the stratum corneum in aged persons is equivalent to that of young skin (Lock-Andersen J et al, 1997) with little difference in the constitutive TEWL (Ghadially R et al, 1995). This shows that for ageing, the TEWL is not a complete evaluation of the functionality of the cutaneous barrier and the epidermal homeostasis of the skin of a subject.
In contrast, the majority of clinical studies described up to now have been based on TEWL measurements in order to evaluate the effect of treatments on the cutaneous barrier function or to monitor the return of the epidermis to a normal homeostatic condition after a challenge. During one study aimed at determining the action of various cosmetic treatments on the barrier function of the human epidermis, transcriptomic analysis of the response of the epidermis and TEWL measurements were carried out simultaneously (Marionnet C et al, 2004). This identified reproducible markers which were common to various challenges, as well as markers which were specific to each challenge; however, the TEWL could not supply elements which could compare the cosmetic treatments which were carried out.
Under these conditions, there is a need for pertinent, reproducible and significant markers to be identified which reflect the epidermal homeostasis and which can identify any imbalance in the barrier function due to the epidermal homeostasis of the skin such as the imbalance due to age.